The sensor measures stresses and/or strain in the layer induced by the force applied to the load-carrying member. The force applied to the load-carrying member is, for example, a tensile force, a compressive force, or a torque, and the load-carrying member is, for example, a metallic shaft. Torque sensors of the type described above are well known in the art, for example, from EP0309979. The purpose of the load-carrying member is to transfer load to the stress-measuring layer.
A magnetoelastic, also denoted magnetostrictive, material, is a material that changes its permeability when it is loaded by a force. Examples of magnetoelastic materials are iron, nickel, cobalt and rare earth metals or alloys thereof.
A hard magnetic material stays magnetized when it has been exposed to a strong magnetic field. A soft magnetic material can not stay magnetized after it has been exposed to a strong magnetic field. A soft magnetic material differs from a hard magnetic material in that it cannot maintain a static magnetic field after it has been magnetized. A hard magnetic material typically has a coercive force over several thousand A/m. A soft magnetic material has a considerably lower coercive force, typically less than 1000 A/m.
There is a desire to be able to measure mechanical stresses in a large load range. For example, in the car industry there is a desire to measure torque-induced shear stresses of a magnitude up to 200-300 MPa. Further, there is a desire to find a torque-measuring device that is long-time stable due to resistance against mechanical and thermal fatigue and is linear, i.e. the output signal from the measuring device is essentially proportional to the load on the load-carrying member. Further, there is a desire to reduce or even eliminate creeping in the output signal from the measuring device, i.e. the output signal should not change its value at a constant load. Hysteresis in the output signal should be avoided as it increases the measurement errors. A dense layer and a good adhesion between the layer and the load-carrying member are prerequisites for achieving a measuring device that is long-time stable, has a large load range, and low hysteresis.
Magnetoelastic effect is explained in terms of magnetic domain wall movement (see for example Modern Magnetic Materials by Robert C. O'Handley Chapter 7 ISBN 0-471-15566-7), thus shape and size of magnetic domains and mobility of magnetic domain walls is of essential importance while building sensors based on the magnetoelastic effect.
Magnetic domains, areas with uniform magnetization direction, have boundaries defined as magnetic domain walls. Magnetic domain walls can move in the magnetoelastic material depending upon the magnetization direction of the material. The walls can move freely in the homogeneous single crystal material where it does not experience any obstacles since single crystal is structurally and chemically homogenous. In amorphous material magnetic domain walls do not experience any obstacles since structural or chemical variations are much smaller than domain wall thickness in such materials. Typically, magnetic domain wall thickness in the soft magnetic materials (for example NiFe or FeCo) is of the order of several hundreds of nanometers, in extreme cases up to 1 μm, hence grain sizes less than 100 nm cannot form an effective obstacle for domain wall. Magnetic domain shape and size in such materials depends on demagnetizing field and shape of the magnetized object trying to minimize energy of the magnetized system.
WO2007/106024 discloses a method for producing a layer on a load-carrying member, which layer is intended for measuring stresses induced by a force applied to the load-carrying member, wherein the method comprises: forming a nanocrystalline layer of a magnetoelastic alloy having an average grain size less than 50 nm on a surface of the member, and heat-treating the layer until a crystallization of the alloy occurs and the average grain size becomes in the range of 100 nm to 10 000 nm. This method drastically improves the stress-measuring properties of the layer, due to the fact that the method produces a layer of a microstructure with a grain size large enough to accommodate one or just a few magnetic domains. A nanocrystalline layer having an average grain size less than 50 nm provides favorable conditions for crystallization and tailoring the mentioned microstructure. Microstructures with grains larger than 10000 nm tend to have higher magnetoelastic sensitivity, which is not favorable when trying to achieve the above-mentioned wide measuring range.
The layer is preferably formed on the member by means of electroplating since electroplating is a suitable method in order to achieve a nanocrystalline layer of the desired grain size. The document mentions that it is also possible to use other methods such as PVD (Physical Vapour Deposition) methods, CVD (Chemical Vapour Deposition) methods, and metal spraying, for applying the layer on the load carrying member. However, there are some disadvantages with the electroplating method. One disadvantage is that it takes a very long time to apply thick layers. The above mentioned layer on a load-carrying member is suitably thicker than 30 μm, and preferably between 100 μm and 300 μm. To apply such a thick layer with electroplating takes hours. Accordingly, electroplating is not a commercially attractive method for applying the layer on the load-carrying member. A further disadvantage with electroplating is that it can lead to a reduced strength of the material on which the layer is applied, and in particular for hardened materials, such as carburized steel.
It is important that the application of the layer is fast and easy, hence economically feasible. Layer application with atom-by-atom methods, such as physical or chemical vapour deposition, are slow and have limitations when building layers tens or hundreds of microns thick. A powder metallurgical method, such as thermal spraying, laser cladding, and sintering, has an advantage of being extremely fast in building up the layer thickness and does not impair the strength of the load carrying layer.
High velocity thermal spray techniques are coating processes in which a powder formed of particles is sprayed onto a surface. The powder can be heated before applying. The powder is typically fed into a spray gun, where it may be heated while being accelerated towards the material to be coated. As the sprayed particles impinge upon the surface, they cool and build up a structure forming the coating. The distinct feature of the powder metallurgical methods, while forming metallic layers, is the inhomogenity of the produced coating due to particles that remain fully or partly melted in the coating. Commonly, the surfaces of the particles in the powder are covered with an outer layer of another material, most often oxide of the metal. The outer layers of the particles are included in the coating and thereby contribute to the inhomogenity of the produced coating. A well-known problem with many powder metallurgical methods is oxidation due to the heating of the powder and the fact that the metal powder is in contact with air during the spraying.
Due to the inhomogenity of the produced coating and relatively high temperatures upon deposition of the layer, high speed thermal spray techniques are not suitable to achieve a homogeneous nanocrystalline layer of a desired uniformity of chemical composition and grain sizes. Thus, high velocity thermal spray techniques cannot be used to produce the homogeneous nanocrystalline layer needed for carrying out the method described in WO2007/106024.
U.S. Pat. No. 6,465,039 discloses a method for forming a magnetostrictive composite coating on a shaft. A powder mix of magnetostrictive rare-earth-iron compound (REF2) particles and matrix metal particles carried in a spray gas stream are applied on the shaft by means of low temperature, high velocity spraying. The temperature of the gas may vary between 300-1000° C. The matrix metal particle are sprayed in particle size ranges from 63 to 90 μm and magnetostrictive rare earth iron particles are sprayed in particle sizes between 63 to 106 μm for shafts. The coating itself includes hard magnetized material, thereby generating and maintaining a static magnetic field in the coating. The shaft with the coating is used in a sensor for measuring torques applied to the shaft. The static magnetic field is redirected in dependence on the torque applied to the shaft, due to changes in the permeability of the coating.
For the successful operation of the magnetoelastic sensor described in U.S. Pat. No. 6,465,039, it is required to have magnetically hard materials with a coercive force over several thousands or even tens of thousands of A/m, which is not necessary to have in the case of the magnetoelastic sensor working on the principle of a time-varying magnetic in the layer inducing a voltage in a measuring device in order to detect changes in the permeability.